<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .box {
            display: flex;
            background-color: pink;
            padding: 10px;
        }

        .box-cell {
            flex: 1;
            background-color: skyblue;
            margin: 5px;
        }


        .box-cell.u-1of3 {
            flex: 0 0 33.3333%;
        }

        .box-cell.u-1of4 {
            flex: 0 0 25%;
        }
    </style>
</head>

<body>
    <div class="box">
        <div class="box-cell u-1of4">...u-1of4</div>
        <div class="box-cell">...</div>
        <div class="box-cell u-1of3">...u-1of3</div>
    </div>
</body>

</html>